This Program encompasses the work of seven projects, all relating to the mechanisms and consequences of interactions among cells with special emphasis on chemical synapses and on the development of their neurotransmitters and transmitter receptors. Among the questions under study are: What is the transmitter that mediates the late slow excitatory postsynaptic potential in amphibian sympathetic neurons; is it a peptide; what is its mechanism of action? Can direct evidence be obtained for a change-over of single rat sympathetic neurons in culture from adrenergic to cholinergic; what is the time course of this change-over; what factors influence the presence and spatial distribution of cholinergic receptors on autonomic neurons? What biochemical changes (membrane proteins, cyclic nucleotides, etc.) accompany the change-over from adrenergic to cholinergic function? What are the mechanisms by which serotonin and octopamine produce striking effect in the function of presynaptic endings and muscle fibers in lobster exoskeleteral muscle? What is the importance of cellular interactions in directing the development of the nervous system in a metamorphosing insect, Manduca; what biochemical changes accompany this development? What is the detailed pattern of synaptic connections between muscle afferents and motoneurons in bullfrog spinal cord; how does his pattern develop; how is this development affected by alterations in peripheral targets? It is the overall aim the Program to learn about the interactions of excitable cells: the rapid and intermediate rate interaction that underlie synaptic transmission and the longer-term interactions important to neuronal development and synapse formation.